Dynamical forcings in heavy precipitation events over Italy: lessons from the HyMeX SOP1 campaign

Abstract

The first Special Observation Period (SOP1) of HyMeX (Hydrological cycle in the Mediterranean eXperiment) was held in fall 2012 and focused on heavy precipitation events (HPEs) and floods in the northwestern Mediterranean. Nine intensive observation periods (IOPs) involved three Italian target areas (northeastern Italy, NEI; Liguria and Tuscany, LT; central Italy, CI), enabling an unprecedented analysis of precipitation systems in these regions. In the present work, we highlight the major findings emerging from the HyMeX campaign and in the subsequent research activity over the three target areas by means of conceptual models and through the identification of the relevant recursive mesoscale features. For NEI, two categories of events (Upstream and Alpine HPEs) were identified, which differ mainly in the temporal evolution of the stability of the upstream environment and of the intensity of the impinging flow. The numerical simulation of convection in the Po Valley was found to be very sensitive to small changes in the environmental conditions, especially when they are close to the threshold between “flow-over” and “flow-around” regimes. For LT, HyMeX SOP1 focused on orographically enhanced precipitation over the Apennines and quasi-stationary mesoscale convective systems over the sea or close to the coast. For the latter category of events, associated with the majority of the recent HPEs, local-scale or large-scale convergence lines appear fundamental to trigger and sustain convection. These lines are affected not only by the orography of the region, but also by the perturbations induced by Sardinia and Corsica on the environmental flow, and, at later times, by cold pools formed via evaporation of precipitation. For CI, a high low-level moisture content and marked low-level convergence over the sea were critical to support deep convection in the IOPs affecting the Tyrrhenian coast. For the HPEs affecting the Adriatic regions, a cut-off low over the Tyrrhenian Sea induces intense bora over the Adriatic basin. Low-level convergence triggers convection over the sea, while orographic uplift produces stratiform precipitation. The Adriatic Sea plays a critical role mainly through air–sea exchanges, which modify the characteristics of the flow and in turn the effect of the orographic forcing.